There have been recently and extensively investigated white light emitting diodes each provided by combining a blue light emitting diode element and a blue-light absorbing/yellow-light emitting phosphor, and have been proposed and presented in various literatures including patent-related references (see patent-unrelated reference 1, and patent-related references 1 through 5, for example). Recently, lighting instruments, lighting equipments, and lighting apparatuses based on the aforementioned combination have been put into practical use.
Known as one of phosphors so frequently used in the above applications, is a yttrium/aluminum/garnet based phosphor activated by cerium, represented by a general formula (Y, Gd)3(Al, Ga)5O12:Ce3+. However, the white light emitting diode comprising a blue light emitting diode element and a yttrium/aluminum/garnet based phosphor has a feature to emit bluish white light due to lack of a red component, thereby problematically exhibiting deviation in a color rendering property.
Meanwhile, lighting techniques have been diversified in usages, usage schemes, and needs, thereby demanding diversified color tone designs including realization of not only white color having a higher color temperature in the aforementioned lighting techniques utilizing light emitting diodes, but also white colors having various color temperatures such as seen in conventional ordinary lighting instruments, respectively. For example, there has been sought for a white light emitting diode for achieving a white color which provides warmness and is called an “incandescent color”. Under such circumstances, there has been investigated a white light emitting diode including two kinds of mixed and dispersed phosphors, such that a red component lacking in case of a yttrium/aluminum/garnet based phosphor is compensated for by an additional red phosphor.
Such a white light emitting diode has been proposed in a patent-related reference 4 (“white light emitting element”), a patent-related reference 5 (“nitride phosphor and production method thereof”), and the like. However, the above-described problem has not been fully solved even by the inventions proposed in them, which is away from a situation where the needs demanding diversified color schemes and chromaticities are fully dealt with, while such proposals are also insufficient in emission intensity, thereby still exhibiting a problem to be solved. Namely, the invention described in the patent-related reference 4 has a problem in that a red phosphor to be used therein contains Cd, i.e., a cadmium element. In this respect, it has been recently conducted to eliminate cadmium and a cadmium-including compound from usage, based on anxiety about environmental pollution, in a manner to alternatively use a substance free of cadmium. It will be thus desirable to make the same approach also in design of fluorescent material in view of the above consideration, since usage of cadmium appears to be undesirable.
Further, red-light emitting phosphors described in a patent-related reference 5 and exemplarily represented by Ca1.97Si5N8:Eu0.03, are not problematic in that the phosphors include no elements like cadmium which are concerned about environmental pollution, but the phosphors are insufficient in emission intensity, so that further improvement is desired therefor. Moreover, the technical concepts described in the patent-related references 4 and 5 have merely and exclusively referred to realization of white color. Lighting techniques are diversified in usage as mentioned above, and decorative effects are also demanded. Thus, diversified color tones and tints are demanded, thereby in turn demanding various light sources for preparing and realizing color tones therefor. Namely, such needs have not been satisfied by white light emitting diodes only, so that there has been desired realization of light emitting diodes of various intermediate colors. Additionally, it has been also desired to extend a chromaticity range as wide as possible from a desired color tone so as to attain a sufficient color tone expression.
[Patent-unrelated reference 1] M. Yamada et al, Jpn. J. Appl. Phys., vol. 42 (2003), pp. L20-23
[Patent-related reference 1] JP-2900928
[Patent-related reference 2] JP-2927279
[Patent-related reference 3] JP-3364229
[Patent-related reference 4] JP-A-10-163535
[Patent-related reference 5] JP-A-2003-321675